Microparticles and methods for their production

ABSTRACT

Microparticles having a metal-containing core encapsulated in a graphitic shell containing hetero atoms are made by forming, in a liquid medium, colloidal particles containing a metal-oxo species of Fe, Co, Ni and Pd, colloidally stabilized by a surfactant and containing source material of carbon and the hetero atoms. These particles are pyrolyzed in inert gas to yield the microparticles. In an alternative method, silica gel coated particles are formed by colloidally stabilizing particles containing metal species and forming silica at the boundary of the stabilized particle.

FIELD OF THE INVENTION

This invention relates to microparticles having metal-containing coresencapsulated in coatings in the form of shells of graphite or silica,and to methods of making such microparticles.

BACKGROUND TO THE INVENTION

There has been great interest in the incorporation of foreign materialsinto enclosed nano-carbon cages. This interest has been driven by thepotential applications of these filled carbon capsules, which lie inareas as diverse as optical, electronic, storage, magnetic recordingmaterials, and nuclear medicine. In particular, carbon (onion-shell)structures of extreme strength may offer excellent protection to theirencapsulated nanomaterials for applications. In addition, the closepacking structures of the carbon shells do not allow exchange ofmolecules/materials from inner cores to the external environment. Wehave now found that materials of extremely reactive (air sensitive) orhazardous (radioactive) nature can be safely caged in thecarbon-enclosed structures.

The idea of using carbon caged structures as molecular containers datesback many years [1,2] before the actual discovery of fullerene andrelated structures. Research in this area is chiefly driven by thepotential applications of filled nanocapsules in areas such aselectronics (quantum dots and wires), magnetic data storage,xerographics, lubrication, sensors and medicinal materials [3-5]. Sofar, several groups have succeeded in encapsulating various nanosizedmaterials into fullerene structures by either using Krätschmer-Huffmanarc [6,7], laser [8] or electron irradiation methods [9] during thefullerene synthesis or by creating an ‘opening’ in the carbon cagedstructures using chemical methods [10-12] prior to filling. Yields ofthe endohedral metallofullerenes synthesis [8] tend to be low and thereare doubts about stability. Filled polygonal shaped carbon nanoparticlesand nanotubes appear to be more promising from the point of view ofapplications. The techniques currently used to synthesise thesestructures include modifications of Krätschmer-Huffman arc method andmethods involving heat treatment of impregnated microporous carbon [13,14]. It is noted that there is no macroscopic synthetic method yetdeveloped for the preparation of filled carbon onions (quasi-sphericalgraphitic shells) although minuscule amounts were produced throughirradiation of amorphous carbon, nanotubes or nanoparticles with foreignatoms with an intense electron beam [9]. No reported work has been foundon the preparation of total carbon encapsulation of radioactivenanoparticles.

SUMMARY OF THE INVENTION

By this invention, a novel methodology for the encapsulation ofradioactive elements within the carbon cage is provided. This simpletechnique allows production of macroscopic quantities of quasi-sphericalgraphitic/fullerenic shells (carbon onions) that can encapsulatenanoparticles containing radioactive element(s) of a very narrow rangeof particle diameters. We believe the method may offer new routes forsafe handling/disposal of radioactive substances and the carbon coatednano-radioactive products may find applications in medical imaging orradiotherapy.

According to the invention in a first aspect, there is provided a methodof making microparticles having a metal-containing core encapsulated ina graphitic shell containing hetero atoms, including the steps of (i)forming, in a liquid medium, colloidal particles containing a firstmetal in the form of a metal-oxo species, the first metal being selectedfrom Fe, Co, Ni and Pd, the particles being colloidally stabilized by asurfactant and containing in addition to the first metal source materialof carbon and the hetero atoms, and (ii) separating said particles fromthe liquid medium and pyrolyzing them in inert gas to yield themicroparticles having said core and said graphitic shell containing saidhetero atoms encapsulating the core.

The basis of the invention in this aspect are the findings that stablecolloidal particles can be produced containing metal-oxo species of Fe,Co, Ni or Pd, and that these particles can be pyrolyzed to provide ametal-containing core, typically of metal, alloy, carbide or oxide ormixtures of these, encapsulated in a good quality graphitic shell whichcontains hetero atoms which are believed to cause curvature to thecarbon-atom layers of the graphite. Further it has been found that asecond metal, such as a radionuclide, can be incorporated in thecolloidal particles. The first metal, which is at least one of Fe, Co,Ni and Pd, is believed to play a role in catalyzing the formation of thegraphite during pyrolysis. The hetero atom, which should be chosen tobecome chemically bound in the graphite molecular structure, may beselected from N, B, P, S and O, and is most preferably N. The heteroatom is believed to cause curvature of the carbon layers of thegraphitic shell, resulting in an encapsulating coating having few or nodefects or fractures.

The metal-oxo species in the colloidal particle is typically insolublein the medium which contains it. It may be produced by oxidation of asoluble compound of the first metal. Two methods of carrying this outare proposed.

In one particular method, step (i) is performed by forming a solution ofa compound of the first metal in the liquid medium which is a polarsolvent and heating the solution in the presence of the surfactant andan oxidizing agent to convert the compound to the metal-oxo species andform the colloidal particles. In this method, a high boiling point polarsolvent is preferably employed, e.g. one having a boiling point above100° C., or even above 200° C. The oxidizing agent may be for exampleoxygen or a compound of a second metal which becomes incorporated in thecolloidal particles. Such a compound of a second metal is for example anoxo-anion, such as ReO₄ ⁻ or TcO₄ ⁻.

In a second method, step (i) is performed by forming an emulsion havingdispersed phase droplets stabilized by the surfactant and containing adissolved compound of the first metal and oxidizing the compound of thefirst metal to produce the metal-oxo species and form the colloidalparticles. The dispersed phase droplets may include a dissolvedoxidizing agent in the form of a compound of a second metal, asdescribed above.

The colloidal particle includes source material of carbon and the heteroatom, for the formation of the graphitic shell. Thus it is not requiredthat another carbon source, e.g. a gas, is provided during thepyrolysis. The carbon may effect reduction of the metal species, toprovide metal or alloy as the core. The carbon and hetero atom may bepresent in a complex of the first metal. Preferably this metal complexincludes ligands selected from cyanide, isocyanide, cyanate andisocyanate.

In a second aspect, the invention provides microparticles having a corecontaining a radionuclide encapsulated by a graphitic shell. Whenproduced by the methods described above, the core contains in additionto the radionuclide, at least one of Fe, Co, Ni and Pd. Further thegraphitic shell may contain chemically bound hetero atoms such that thegraphitic carbon layers of the shell are curved.

In nuclear medicine, the choice and form of radionuclides should becarefully chosen. The choice of a radionuclide for imaging purposes ischiefly dictated by the necessity of minimizing the radiation dose tothe patient and the detection characteristics of present-day nuclearmedicine instrumentation. The forms should preferably be non-toxic inthe desired amounts, and would not directly involve in the patient'sphysiological body mechanisms. The form, structure and morphology andconcentration of the radioactive material will also affect the imagingquality (small particle size will give better imaging quality i.e.smaller pixels). This invention describes a novel, easy, convenientmethod for the synthesis of complete carbon encapsulated radionuclideswith a narrow particle size distribution suitable for diagnostic imagingand therapeutic applications. The impermeable carbon coatings offertotal isolation of the enclosed nuclides from the environment.

The novel method described herein is concerned with a methodology forthe production of a complete carbon encapsulated radioactive materialwith a small range of particle size. We believe these materials producedfrom the method will find applications in lung ventilation and lungperfusion imaging and other diagnostic and radiotherapy areas.

This method may be suitable to encapsulate a wide range of radionuclides(parental or daughter nuclides) with different emitted radiation anddecay times.

Technetium-99m (^(99m)Tc) has excellent physical characteristics fordiagnostic imaging having a half-life of 6 h and emitting gamma-rayphotons at an energy of 140 keV which are suitable for detection with agamma camera. ^(99m)Tc is easily obtained from a commercially available⁹⁹Mo/^(99m)Tc generator. ⁹⁹Mo is a fission product which is obtainablein a carrier-free form with a high specific activity. It has a half-lifeof 66.02 h and decays to ^(99m)Tc by β− emission, which decays byisomeric transition to ⁹⁹Tc. In commercially available generators ⁹⁹Mois absorbed onto an alumina column. Elution of the generator with 0.9%sodium chloride solution elutes the ^(99m)Tc in the form of sodiumpertechnetate (Na^(99m)TcO₄) leaving the ⁹⁹Mo bound to the column¹⁵.⁹⁹Mo/^(99m)Tc generators are generally purchased by hospitals on aweekly basis and have a working life of one week. ^(99m)Tc isincorporated in a variety of chemical forms for oral intake, injectionor for lung ventilation/perfusion.

For lung imaging “Technegas” has been developed. It is an ultrafine^(99m)TC-labelled aerosol introduced by W. Burch [22] in 1986. The smallradioactive particles (nanometric in size) in a Technegas aerosol resultin better clinical images than those obtained from other radioactiveaerosols. It is known that the size and the content of the Tc particlein the aerosol can critically affect the quality of the images. The sizeof the aerosol particles also determines the site of lung deposition(bronchial and alveolar regions), subsequent modes and rates ofclearance and hence will affect the diagnostic information obtained.More recent work, using electron microscopy, has shown that ^(99m)Tcparticles in Technegas are in fact relatively large crystals of over 100nm in diameter. Another problem that arises with the production ofTechnegas is that significant quantities of ^(99m)Tc are incompletelyprotected in the carbon matrix. This leads to leaching of radioactive^(99m)Tc to body fluids hence resulting in the undesirable appearance of^(99m)Tc in the saliva, oesophagus, stomach and thyroid gland and thusdegrading the quality of the images. In addition the high cost ofpurchasing the commercial Technegas generator has limited its use inmany centres.

Our method in one embodiment involves the preparation ofgraphitic-encapsulated microparticles, more particularly nanoparticles,by the dissolution of a source or sources of iron, carbon and a heteroatom (usually N, but B, P, S or O for example may be used) in a polarsolvent at elevated temperature; partially decomposing the iron compoundto an insoluble iron oxygen species; size controlling and stabilizingthe partially oxidised iron species by use of a surfactant; andpyrolyzing the stabilized species to yield a metallic core encapsulatedby a graphitic/hetero shell. To form a radionuclide-containing particle,a salt or complex of a radionuclide is also present. It is found thatduring the partial decomposition of the iron compound, especially in theabsence of oxygen, the radionuclide is incorporated into the core.

Since the graphitic encapsulation derives primarily from the carbonsource, regulation of the carbon:iron ratio can help determine thenumbers of encapsulating layers around a core; this in turn may act as amodulator or regulator of the radiation emitted from a radionuclide inthe core.

In more detail some of the most important synthesis steps in obtainingthe desired products have been elucidated from study of an iron cyanidecomplex as a starting material. These include the fact that iron species(catalyst) catalyse formation of enclosed graphitic structure from itsattached cyanide ligand (or carbon and nitrogen containing ligand,stabiliser) upon heating, which will subsequently enclose the ironparticle forming completely encapsulated particles. Prior to heattreatment, control in particle size is important in order to makeencapsulated nanosized particles. We therefore show that addingsurfactants or polymers controls the particle sizes and stabilisation ofinsoluble iron and cyanide-containing nanoparticles when soluble ironcyanide species is partially decomposed in a high-boiling polar solvent.This is based on the fact that the polymer/surfactant adsorbs at thesurface of the newly formed nano-particles forming micellar protectionso preventing aggregation. In other contexts, stabilisation of colloidin a polymer solution is a well-established technique typically in paintand ink-jet technologies. Thus, we show that controlled size of Fecyanide containing nanoparticles stabilised with adsorbedpolymers/surfactants is a most important step before the radioactiveelement incorporation. The incorporation of foreign element(s) to thestabilised iron containing particles can be subsequently performed bydeveloping specific interactions (chemical linkage, ion-exchange, redoxattachment, etc) with the Fe(II) containing particles. Finally, afterthe removal of solvents followed by heat treatment the stabilisednanoparticles containing iron and foreign atoms with a narrow sizedistribution will give rise to the desired products. It is noted thatthe residue cyanide species in the particles will provide the carbonsource for the formation of enclosed graphitic structure. We show thatRe and Tc species can be successfully incorporated into thecarbon-encapsulated iron containing nanoparticles via using ReO₄ ⁻ orTcO₄ ⁻. Incorporation of ^(99m)Tc is thus possible. Re and Tc havealmost identical chemical properties (due to the lanthanidecontraction). Other radioactive elements such as ^(99m)Mo, ¹¹³Sn cansimilarly be incorporated into this novel carrier (surfactant stabilisediron(oxo) cyanide nanoparticles) by employing similar chemistry.

The invention thus provides a low temperature solution method forencapsulation of radioactive nano-particles/clusters with a fine controlin particle size. There are many advantages in the safehandling/disposal of radioactive materials if the materials are storedin an impervious carbon coating. Early work suggested it was difficultto carry out complete encapsulation of material with graphitic carbonunless the temperature rose above the vaporisation temperatures ofgraphite (>2500° C.) during electric arc excitation [22]. However, thereare many disadvantages associated with the extremely high temperaturesynthetic method especially regarding to the difficulty in controllingthe particle size. Complete carbon encapsulation over radioactivenanoparticles thus may require a high temperature since amorphous carbonatoms do not offer total protection to these particle against leaching.The crystallisation of amorphous carbon atoms into complete graphiticprotective capsule structures (graphitic carbon shells) is known to takeplace at minimum temperature of 2500° C. [25]. However, the alternativewould be to induce graphitisation at low temperatures by the use ofcatalyst. The phenomenon of low temperature (<700° C.) “catalyticgraphitisation” from a different variety of carbon sources over Fe, Co,Ni, Cr, Pd, has been known for many years. Here we demonstrate clearlythat the formation of iron or iron carbide encapsulated in carbon shellsfrom heat treatments of stabilised iron (oxo) cyanide species can beachieved. The cyanide species provide carbon source for the formation ofenclosed graphitic shells. This indicates an effective catalyticcarbonisation over the iron materials.

In summary, we report the (a) effectiveness of using catalytic component(Fe or alternatively Ni, Co or Pd, e.g. as cyanides) to formwell-defined sized nanoparticles in a high boiling polar solvent (withstabiliser) during their cyanide decomposition; (b) incorporation offoreign atoms (radioactive atom, such as ^(99m)Tc) into the ironoxo-cyanide nanoparticle aggregates can be achieved using the oxidative^(99m)TcO₄ ions through a fast redox trapping mechanism; (c) thermaldecomposition of these colloidal particles (with cyanide) result in theenclosed graphitic structure embracing radioactive element(s).

In another aspect, the invention provides the formation of silica-coatedmicroparticles, by a similar method. In this aspect, the inventionprovides a method of making solid microparticles having ametal-containing core surrounded by a silica coating, including thesteps of

-   (i) forming, in a liquid medium, colloidal particles containing a    metal-containing species and colloidally stabilized by a surfactant,    and-   (ii) forming a silica coating around said colloidal particles by    hydrolyzing a silicon compound in the region of the interface    between the colloidal particle and the liquid medium.

A plurality of metal-containing species of different metals may beincluded in the colloidal particles, and thus in the core of themicroparticles produced. Typically said metal-containing species isselected from metal, alloy, metal oxide, metal hydroxide and metalcarbide. Preferably the metal-containing species is ferromagnetic(enabling magnetic separation of the microparticles from liquid) and/orcontains a radionuclide.

Preferably the microparticles are aged, e.g. for a day or more, beforeremoval from the system containing the silicon compound, in order toestablish the silica coating to the desired thickness. The coating maybe porous.

In a preferred form of the method in step (i) the colloidal particlesare made by forming an emulsion having dispersed phase dropletsstabilized by the surfactant and containing a dissolved compound of themetal and causing the metal-containing species to precipitate therebyforming the colloidal particles. The precipitation may be caused byaddition of alkali.

The silicon compound which is hydrolyzed may be an alkoxy silanecompound, i.e. a compound containing at least one Si—OR linkage, where Ris alkyl of preferably 1-8 carbon atoms, more preferably 1-4 carbonatoms, such as tetraethyl ortho silane (TEOS, Si(OC₂H₅)₄).

Further the invention provides microparticles each having a corecomprising at least one metal-containing species which is ferromagneticand/or contains a radionuclide and a coating of silica gel encapsulatingthe core, which may be produced by the above method. The silica gel mayhave at its surface functional groups, e.g. OH groups, for theattachment of other species, such as biochemical or biological species(e.g. peptides, markers, cognate binding partner, solubilizers).

The cores of the silica-coated microparticles preferably have an averagediameter in the range 1 to 100 nm, more preferably 1 to 50 nm. Thesilica gel coating may have any desired thickness, but preferably has anaverage thickness in the range 1 to 50 nm, preferably 2 to 10 nm, e.g. 2to 4 nm.

By this method, ferromagnetic cores and/or radionuclides can beencapsulated in silica coatings.

BRIEF INTRODUCTION OF THE DRAWINGS

In the drawings:

FIG. 1 is an electron micrograph showing a large discreteiron-containing particle encapsulated within multi-layer graphiticcarbon shell prepared from decomposition of iron cyanide particle innitrogen (scale bar=50 nm);

FIG. 2 shows the powder X-ray diffraction (XRD) spectrum of pureFe^(III) ₄[Fe^(II)(CN)₆]₃ salt acting as a precursor;

FIGS. 3 and 4 show the powder X-ray diffraction (XRD) spectra of partialair decomposed Fe^(III) ₄[Fe^(II)(CN)₆]₃ in refluxing dioctylether (FIG.3: small amount of dissolved oxygen in dioctylether; FIG. 4: oxygencontinuously purging through the system);

FIG. 5 (a TEM micrograph) shows the colloidal stable iron-oxo-cyanidecontaining nanoparticles obtained via refluxing 0.86 g Fe^(III)₄[Fe^(II)(CN)₆]₃ salt in 40 ml dioctylether solvent with addition ofoleic acid (20 ml). Small but homogeneous sized organic stabilisednanoparticles (−10 nm) are clearly visible (nanosize particles giving asuper-lattice packing);

FIG. 6 a is a typical high resolution transmission electron micrograph(lattice image) showing iron nanoparticles with the interplanar spacingof 0.21±0.05 nm corresponding to (111) of fcc Fe (cementite)encapsulated in concentric quasi-spherical graphitic shells of 3.4×10⁻¹⁰m spacing (scale bar=5 nm);

FIG. 6 b is a TEM showing the same product as FIG. 6 a on a larger scale(scale bar=1 nm);

FIG. 7 is the EDX spectrum of Re-incorporated stabilised Fenanoparticles which show the selected area rich in iron and rhenium (Cupeaks arise from the copper grids used for support);

FIG. 8 shows the corresponding high-resolution TEM image ofRe-incorporated stabilised Fe nanoparticles with very well defined sizeand shape (as FIG. 7) (scale bar=20 nm);

FIG. 9 a is a high resolution TEM image of a spherical graphitic shellstructure (2-5 layers) filled with Fe and Re containing nanoparticlesafter applying heat treatments to the particles of FIG. 8 (scale bar=20nm);

FIG. 9 b is a high resolution TEM showing the same product as FIG. 9 aon a larger scale (scale bar=5 nm), with inset EDX spectrum of theselected area showing Re and Fe in the particle core (the Cu peaks arisefrom copper support grids);

FIG. 10 shows the result of irradiation of amorphous carbon with intenseelectron beam in TEM showing it to have spherical graphitic concentricshells (onions); and a diagrammatic model attached (scale bar 100×10⁻¹⁰m);

FIG. 11 is a high resolution TEM showing a spherical graphitic shellstructure filled with a nanoparticle containing Fe and ^(99m)Tc producedin Example 4;

FIG. 12 is a bar chart of activity counts obtained in the proceduredescribed in Example 4;

FIG. 13 is the XRD spectrum of the iron oxide core nanoparticles ofExample 5;

FIG. 14 is the XRD spectrum of the iron cobalt oxide core nanoparticlesof Example 6;

FIG. 15 is the EDS analysis for the iron oxide core nanoparticles ofExample 5;

FIG. 16 is the EDS analysis for the iron cobalt oxide core nanoparticlesof Example 6; and

FIG. 17 is a TEM micrograph of the particles of Example 5 (scale bar=5nm).

EXPERIMENTAL DETAILS AND RESULTS Example 1 Synthesis of CarbonEncapsulated Iron Particles (Comparative)

A sample of iron (III) ferrocyanide (purity>99.9%) obtained from Sigmaplc, was transferred into a quartz tube with one end plugged with quartzwool. The filled tube, which was placed in the tube furnace, was firstheated to 200° C. (heating rate: 5° C./min, duration: 2 h), and thenfinally to 900° C. (heating rate: 10° C./min; duration: 3 h) in N₂atmosphere (approx. flow rate: 0.9 l/min). The black materials formedwere stored in a sample vial. Elemental analyses from the EDX analysisand microanalysis showed that the sample after the heat treatmentcontained mainly C, Fe and a small amount of nitrogen (2-4%), which wasscratched off the tube and examined by TEM. The TEM micrograph in FIG. 1shows that an iron-containing particle (iron and iron carbides) isencapsulated by graphitic carbon shells (onion structure) though thesizes of most of these encapsulated particles are very large (>0.1 μm).Thus, it is clear that iron catalyses formation of enclosed graphiticstructure from cyanide- or carbon-nitrogen-containing ligand. Othermetals such as alkali or alkaline earths are unable to catalyse theseenclosed graphitic structures from their cyanide salts, nor are ironspecies without using nitrogen containing salt/ligand/stabiliser.

Example 2 Synthesis of Carbon Encapsulated Iron Nanoparticles

Example 1 discloses the use of iron cyanide salt for the formation of anenclosed graphitic structure embracing an iron particle, but no controlin particle size was achieved. Here we describe the use of a polar highboiling solvent (such as dioctyl ether of boiling point 287° C.) todissolve the iron ferrocyanide compound at refluxing temperature (thecompound is fairly soluble in the solvent at elevated temperature)giving intense blue colour solution. Iron-containing cyanide compoundsare known to decompose at about 200-250° C. [23, 24]. In the presence ofdissolved oxygen (air) achieved by purging the system with an airstream, the Fe^(II)(CN)₆ ⁴− of the Fe^(III) ₄[Fe^(II)(CN)₆]₃ saltdecomposes to Fe(CN)₂₋₃O₁₋₂ ^(n−) losing the cyanide species butsimultaneously replacing them with oxygen species [23]. This willultimately lead to precipitating of iron oxide that is insoluble in thesolvent. FIG. 2 shows the powder X-ray diffraction (XRD) of the pureFe^(III) ₄[Fe^(II)(CN)₆]₃ salt. Iron oxide structure (magnetite, Fe₃O₄)is formed from the partial iron cyanide decomposition by air, givingmixed phases in the XRD patterns. The extent of the decompositiondepends on the oxygen availability and duration of the treatment (FIGS.3 and 4). An organic surfactant/stabiliser, such as oleic acid orpolyvinylpyrrolidone (PVP), stabilises the partially oxidised ironcyanide species colloidally against precipitation from the solution.FIG. 5 (TEM micrograph) shows the colloidal stable iron-oxo-cyanidecontaining nanoparticles obtained via refluxing 0.86 g Fe^(III)₄[Fe^(II)(CN)₆]₃ salt in 40 ml dioctylether solvent with adding oleicacid (20 ml). Small but homogeneous particle size (nanosize particlesgiving a super-lattice packing) is obtained using this preparativemixture. There is a clear indication that the size of the particle canalso be finely controlled by tailoring the ratio of oleic acid to theiron compound. Oleic acid, having a polar head group (acid group) and adouble bond in the middle of the molecule, acts as surfactant. It isenvisaged that the double bond will interact strongly with the ironspecies in the inner core of a micelle while its polar head will faceoutwards forming a normal phase micelle in this polar solvent. Hence themicellar structure provides a fine control in particle size. Applicationof the same heat treatment as mentioned in Example 1 to the colloidaliron containing particles of this example produces iron/iron carbideparticle encapsulated in the enclosed graphitic carbon shelled structure(see FIGS. 6 a and 6 b and compare the model view in FIG. 6 c).

Example 3 Synthesis of Carbon Encapsulated Nanoparticles Containing Ironand Rhenium

Example 2 shows that molecular oxygen from air can partially oxidise theiron cyanide compound in dioctylether at elevated temperatures. Adding asmall quantity of sodium perrhenate (inorganic oxidant) to the mixture,as described in Table 1, can also oxidise the iron (II) cyanide species.Experimental details for typical synthesis are described below: TABLE 1The starting materials. All the materials were used as supplied byAldrich. Act. Chemical Name Chemical Formula Mol. Wt. Wt/g FeaturesSodium NaReO₄ 273 0.28 White Perrhenate Crystals Iron (III) Fe^(III)₄[Fe^(II)(CN)₆]₃ 859.25 0.86 Blue powder Ferrocyanide Dioctylether[CH₃(CH₂)₇]₂O 242.45 40 ml Clear Liquid Oleic acidCH₃(CH₂)₇CH═CH(CH₂)₇COOH 282 20 ml —

The chemicals shown above, see Table 1, were mixed in a 3-necked roundbottom flask using a magnetic stirrer bar. Before the mixture was heatedup to 290° C. for 18 h, a gentle stream of nitrogen gas was bubbleddirectly into the mixture for 30 min. Nitrogen gas was continuouslybubbled into mixture during the reaction. The black mixture formed wasallowed to cool to room temperature. To separate the particles, 20 ml ofethanol was added to the product before the resultant mixture wascentrifuged at 4000 rpm for 20 min. This procedure was repeated 3 times.The blue resultant solids were allowed to air dry in the fume cupboard.

Carbon encapsulation is achieved by heating the particles in nitrogen asfollows. The as-synthesised sample was first transferred into a quartztube with one end plugged with quartz wool. The filled tube, which wasplaced in the tube furnace, was first heated to 200° C. (heating rate:5° C./min, duration: 2 h), and then finally to 900° C. (heating rate:10° C./min; duration: 3 h) in N₂ atmosphere (approx. flow rate: 0.9l/min). The black materials were stored in a sample vial.

After heat treatment (which may be carried out in various stages orsteps) the closed graphitic structures are observed embracing both theiron and rhenium elements within the carbon enclosed shells (FIG. 9).

Hence, during the decomposition of iron cyanide at elevated temperature,in the absence of air, the oxygen species will be transferred from theReO₄ ⁻ species to the Fe^(II)(CN)₆ ⁴⁻ (Re(+7) will be reduced whilesimultaneously oxidising the Fe²⁺) leading to incorporation of the Respecies into the stabilised very well-defined, nano-sized iron (oxo)cyanide particles (see EDX analysis in FIG. 7 and the TEM micrograph inFIG. 8 prior heat treatments).

Because of the d− electron configuration (lanthanide contraction)rhenium (Re) shows almost identical chemical properties as technetium(Tc). Typically, the reduction potentials of the ReO₄ ⁻ and ^(99m)TcO₄ ⁻species are about 0.7 V (V versus NHE). It is reported that a fast rateof reductive deposition of TcO₄ ⁻ is obtained over small solid magnetite(iron oxide). ^(99m)TcO₄ ⁻, through its reductive deposition onto theorganic stabilised nano-iron containing particles as described in thisprocess, followed by heat treatment, will make a carbon encapsulatedparticle containing radioactive ^(99m)Tc element. As a result, the smallbut defined dimensions of the iron core (as a catalyst) can incorporatea significant amount of radioactive ^(99m)Tc into the final carbonencapsulated particles for imaging, storage and radio-therapeuticapplications by this technique.

Example 4 Synthesis of Carbon Encapsulated Nanoparticles containing Ironand Technetium

The procedure of Example 3 was repeated, using Na⁹⁹TcO₄ in place ofNaReO₄. The technetium was incorporated into the colloidal particlesformed and appeared in the pyrolyzed graphitic shell particles. FIG. 11is a TEM image of the spherical graphitic shell nanoparticles filledwith Fe and ^(99m)Tc in the final product. FIG. 12 shows activity countsat different stages of the procedure, as now described. Activity assayswere evaluated using gamma counter: 1 mL of ^(99m)TcO₄ ⁻ of 5.19×10⁻¹¹mole dm⁻³ as the standard. This solution added to the mixture asdescribed and allowed to reflux for 1 h (˜1 mL water collected with noradioactivity). Ethanol addition lead to precipitation as 1^(st) pelletand the supernatant as 1^(st) Sup. Repeated treatments produced 2^(nd)Sup and 2^(nd) pellet. After 1000° C. for 1 h the 2^(nd) pellet producedsolid as carbon. This sample washed with 4M HNO₃, the supernatant asAcid Washing and the solid as Carbon Washed. The radioactivity assaysclearly suggested that there had been 13% loss radioactivity due to thephysical transfers (powder sticking on reactor tubes), only 1% loss wasdue to extensive acid treatment (this standard acid digest withsonication would remove externally bound ^(99m)Tc, if any). As a result,the overall radioactivity retention is about 77% in the non-acidleachable solid sample. The surprisingly low activity in the acid washsolution suggests that majority of the particles were chemicallyprotected with acid impenetrable graphitic jackets as indicated from theTEM micrographs.

Characterisation of the Product:

High resolution TEM micrographs indicate that the iron-containingparticles were mainly single nano-crystals of γ-Fe metal (confirmed byXRD) although Fe₃C crystals as the inner cores were sometimes observedfrom the TEM lattice imaging. Detailed examination of thequasi-spherical carbon structures showed that in many cases, carbonlattice fringes (about 0.34 nm) could be traced, continuously yielding,quite surprisingly, hollow concentric carbon shelled structures. Noprolonged exposure of the selected area to the electron beam was ensured(<60 seconds); hence the possibility of onions structure formation dueto electron beam illumination is rejected. We found some completefilling but more frequently partial filling of the encapsulated ironparticles to the carbon cages and there was no obvious preferred latticefringe orientation with respect to the particle and the carbon layers.It is interesting to note that these highly ordered quasi-sphericalconcentric graphitic shells structures produced from iron cyanidedecomposition are clearly different from tubular or polyhedral carbonstructures. Formation of tubular/filamental graphitic carbons is wellknown to occur when a carbon source (i.e. hydrocarbon gas) is in contactwith iron particles at high temperatures [15]. Polyhedral nanoparticlesof central cavities of varying sizes were also observed when amorphouscarbons were exposed to high temperatures [16]. It is also noted thatextremely small quantities of γ-Fe nano-metal crystallites encapsulatedin polyhedral nanoparticles [17] were produced in a large amount ofcarbonanceous debris using the ‘stuffed anode’ modifiedKrätschmer-Huffman method where significant quantities of carbonnanotubes or ‘sea urchin’ structures [18] (carbon nanotubes growradially from the metal nanocrystal) were also found. None of thesestructures were however seen in our sample. We note that ourquasi-spherical graphitic shell structures rather closely resemble tothe ‘carbon onions’ (see present FIG. 10) reported by Ugarte who appliedan intense electron beam irradiation on carbon nanotubes for theconversion [19], although structurally less perfect than his. Theperfect onion is thought to compose of giant fullerenes giving perfectconcentric shells with a quasi-spherical structure. Subsequent work [20]demonstrated ‘carbon onions’ are highly unstable when not beingirradiated in an electron beam and will collapse into a disordered,quite often faceted, configuration, though still with a spheroidalstructure somewhat akin to the structures observed.

Here, we have shown that heating the iron(oxo) cyanide nanoparticles(with or without foreign atoms) at 900° C. results in graphitisation andthe formation of many iron containing nanoparticles encapsulated inquasi-spherical graphitic shell. Presumably, the iron nanoparticles wereformed from the reduction of iron oxide with the nearby cyanide sourceduring the carbonisation process.

The phenomenon of ‘catalytic’ graphitisation is a well-known one [11],but the mechanism in which graphitisation is promoted by the presence ofa second phase is not well understood. In some cases, it is believedthat the carbon is dissolved in the metal or metal carbide andre-precipitated as graphite. In other cases the metal or metal carbideparticles may simply act as templates for the epitaxial growth ofgraphite. However, in all these cases, only long graphitic tubular orfilamental forms of carbons and polyhedral nanoparticles are formed. Theexclusive formation of highly ordered quasi-spherical graphiticnanocapsule structures indicative of ‘fullerene-like structure’ in ourcase however, has not been reported. It is believed that some nitrogen(residue of cyanide decomposition) resulting from the decomposition ofcyanide species may play an important role in self-assembling the carbonatoms into corresponding quasi-spherical structure under our reactionconditions. Nitrogen (non-sp2) adopts different structure from carbon(sp2). Hence its incorporation into carbon graphene layer (all C is insp2 hybridization) is thought to be responsible for the curvature of thegraphitic planes resulting in the particle encapsulation.

The present method for the formation of enclosed graphitic carbonstructure embracing iron nanoparticle at relatively very mildtemperatures (900° C.) is unprecedented. Thus, iron filled sphericalcarbon nanocapsules of a very narrow size distribution in macroscopicquantities by the controlled decomposition of cyanide-containing speciescould be prepared. Prior to pyrolysis, use of surfactant/polymer asstabiliser could provide a fine control of particle size. This is basedon the observation that the polymer/surfactant adsorbs at the surface ofthe newly formed nano-particles through the formation of surface activemicellar aggregates so preventing aggregation of nanoparticle in thesolvent. The internal space is filled with an inorganic iron (oxo)cyanide nanoparticle aggregates with defined dimensions which are inturn controlled by the micellar dimensions. Further immobilisation ofother foreign atoms onto the nanoparticle aggregates can be carried out.Here, we show that elemental Re element or Tc can be incorporated intothe final product. Thus, the controlled carbonisation of these ironcontaining nano-assemblies provides the novel carrier which could openup a new avenue for preparing carbon nanocapsules filled with specificradioactive element(s).

There are considerable benefits of forming total carbon encapsulatedradionuclides that are well suited for medical diagnosis and therapeuticpurposes. The potential beneficiaries will be to patients with lungdisease using the carbon encapsulated ^(99m)Tc—Fe nano-particlesdescribed herein. For example, pulmonary embolism is a major cause ofmorbidity and mortality. It is estimated to account for approximately21,000 deaths annually in England and Wales and over 200,000 in the USA.Early treatment by anticoagulation could save life, but it can sometimesbe hazardous; a reliable means of diagnosis is therefore essential. Thenew technology for producing diagnostic or therapeutic material couldprovide rapid and efficient diagnosis suitable for patients with this orother pulmonary disease, hence improving life expectancy and quality.

For Examples 5 and 6, iron (II) chloride tetrahydrate, iron (III)chloride hexahydrate, cobalt (II) chloride hexahydrate, tetraethylorthosilicate (TEOS), cetyl trimethyl ammonium bromide (CTAB) wereobtained from Aldrich in analytical grade quality and used withoutfurther purification.

Example 5 Preparation of the Silica Coated Magnetic Nanoparticle

Formation of an aqueous microemulsion was carried out using de-ionizedwater, in organic solvent (i.e. dioctyl ether) and surfactant(s) (CTABor oleic acid). As the size of such a micelle system is related to theratio water/surfactant, a small amount of water to surfactant was usedin order to reduce the size of the final nano-composite synthesized. Theexperiment was carried out in room temperature. The microemulsion wasformed as follows: 6.0810 g (0.01668 mol) CTAB was added into the 120 mldried organic solvent under vigorous stirring. After a well-distributedsuspension of the surfactant in the solvent was achieved, 4.3 ml aqueoussolution containing 0.3428 g FeCl₂.4H₂O and 0.9321 g FeCl₃.6H₂ O wasadded slowly in droplets into the suspension of the surfactant insolvent with nitrogen bubbled for two hours to avoid the possibleoxidation of particles. After stopping of pumping of nitrogen, thesystem was stirred overnight to form the microemulsion. Then, nitrogenwas bubbled again and 1.2 ml 18.1M NH₃.H₂O was placed in the path inwhich the N₂ gas flowed. One hour after the whole reaction system hasturned black, 6.9351 g TEOS was added into the reaction mixture.Nitrogen was continuing bubbled through this microemulsion for one morehour. Formation of the silica-gel coating can be achieved because theexcess ammonia catalyzed hydrolysis/condensation of the organic solubleTEOS on the surface of the magnetic oxide nanoparticle (once in contactwith water at the interface between the microemulsion and the bulkorganic solvent). The envisaged chemical reactions are:

Ageing may take some time (i.e. 5 days) to form the silica layer fully.Then, when 30 ml ethanol was added to the microemulsion system,precipitates of silica coated magnetic oxides were rapidly deposited bymagnetic separation using an external magnetic field. The solvent wasremoved followed with an addition of another 30 ml ethanol into theprecipitates and reflux the mixture for overnight to remove thesurfactant from the precipitates. The precipitates were isolated againby magnetic separation and the solvent was removed. Ethanol, water andacetone wash of the precipitates several times. Finally the precipitateswere dried in room temperature and a deep brown powder was collected forfurther characterization. These precipitates showed very strong magneticforce.

Example 6 Synthesis Iron Cobalt Oxide Core Nanoparticle

The same microemulsion system as in Example 5 was applied to synthesizethe iron cobalt oxide core nanoparticles. 6.0810 g (0.01668 mol) CTABwas added into the 120 ml dried organic solvent under vigorous stirring.After a well-distributed suspension of CTAB was achieved, 3.8 ml aqueoussolution contained 0.4056 g CoCl₂.6H₂O and 0.9216 g FeCl₃.6 H₂ O wasadded slowly in droplets into the suspension of CTAB in organic solventwith nitrogen bubbled two hours for avoiding possible oxidation of theparticles. After stopping of pumping of the nitrogen, the system wasstirred overnight to form the microemulsion. The system was slowlyheated to 65° C. and maintained at that temperature. 1.7 ml 10 M NaOHsolution was added with nitrogen bubbled again. One hour after the wholereaction system turned black, it was cooled down, and 6.9351 g TEOS wasadded into the reaction mixture. Nitrogen was bubbled through thismicroemulsion for one more hour. In this case:CoCl₂+2FeCl₃+8NaOH→Fe₂CoO₄(a magnetic oxide phase)+8NaCl+4H₂O

Ageing also took about 5 days to form the silica layer fully. Then, thesame method as above was used to wash and separate the precipitates andcollect them for further characterization. These precipitates alsoshowed very strong magnetic force.

Product Characterization:

For characterizing the two kinds of nanoparticles of Examples 5 and 6,X-ray powder diffraction (XRD) analysis of the samples was performedwith copper Kα₁ of 1.5405 Å radiation. Transmission electron microscopy(TEM) analysis was used to characterize the particle size, structure,morphology and composition through direct imaging, electron diffractionand elemental analysis of selected area (EDS). A Philip CM20 microscopeoperating at 200 kV equipped by an energy dispersive spectrometer (EDS)was used. Samples were gently ground, suspended in isopropanol andplaced on carbon-coated copper grid after the evaporation of thesolvent. Electron micrographs and EDS analysis of selected area weretaken.

XRD spectrum (FIG. 13) shows the nano-scale iron oxide of Example 5.From comparison with published XRD data the composition of the ironoxide could either be Fe₂O₃ or Fe₃O₄ or mixed phases of them since theirlattice parameters would not allow further differentiation. It is notedthat even a pure phase might still have undergone further reactions (airoxidation) when the particles were exposed in air.

The average diameter particle size D was determined by theDebye-Scherrer formula using the half maximum width β of X-raydiffraction lines (D=(κλ)/(β_((hkl))cos θ). In this equation, the D isthe size of nanoparticle, κ is the Scherrer constant (0.9), λ is theX-ray wavelength (1.54056 in our experiments), and the β_((hkl)) is thefull width half maximum (FWHM) of the reflection hkl measured in 20 andis the corresponding Bragg angle. For the powder of Example 5, theaverage particle size is found to be about 17.33 nm.

For the iron cobalt oxide core nanoparticles of Example 6, the XRDspectrum (FIG. 12) confirms the formation of nano-scale iron cobaltoxide. The result was compared with the published XRD database, andshowed that the iron oxide is similar to CoFe₂O₄. According to theDebye-Scherrer calculation the average particle size is about 19.93 nm.

FIG. 15 shows the EDS analysis of the iron oxide core nanoparticles ofExample 5. From the analysis results taking the corrected AFZ intoaccount, we obtain the atomic ratios:

-   -   Fe:Si:=25.39:14.02:60.59.

The calculated formula is Fe₃O_(3.84).1.66 SiO₂ confirming that thepre-dominant phase of the core is Fe₃ O₄ with silica coat(s). In linewith the XRD analysis results, it was found that the average compositionof the particles was indeed between Fe₂O₃ and Fe₃O₄ phase (lying closerto the Fe₃O₄ phase).

FIG. 16 shows the EDS analysis of the iron cobalt oxide corenanoparticles of Example 6. From the analysis results, we obtain theatomic ratios:

-   -   Fe:Co:Si:O=10.11:5.54:22.96:61.40

The calculated formula is CoFe_(1.83)O_(2.79).4.14 SiO₂. Theconcentration of silica seems to be higher when compared with the EDSanalysis of the iron oxide core nanoparticles of Example 5, but thecalculated composition is more or less similar to the phase obtainedfrom the XRD analysis within experimental error.

FIG. 17 shows the particles of Example 5, clearly revealing the metaloxide core surrounded by silica.

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1. Method of making microparticles having a metal-containing coreencapsulated in a graphitic shell containing hetero atoms, including thesteps of (i) forming, in a liquid medium, colloidal particles containinga first metal in the form of a metal-oxo species, the first metal beingselected from Fe, Co, Ni and Pd, the particles being colloidallystabilized by a surfactant and containing in addition to the first metalsource material of carbon and the hetero atoms, and (ii) separating saidparticles from the liquid medium and pyrolyzing them in inert gas toyield the microparticles having said core and said graphitic shellcontaining said hetero atoms encapsulating the core.
 2. Method accordingto claim 1 wherein said metal containing core contains at least onephase selected from metal, alloy, metal carbide and metal oxide and,optionally, is ferromagnetic.
 3. Method according to claim 1 or 2wherein in step (i) at least one second metal is incorporated in saidcolloidal particles and is present in the core of the microparticlesobtained.
 4. Method according to claim 3 wherein said second metal is aradionuclide.
 5. Method according to claim 1 wherein step (i) isperformed by forming a solution of a compound of said first metal insaid liquid medium which is a polar solvent and heating said solution inthe presence of said surfactant and an oxidizing agent to convert saidcompound to said metal-oxo species and form said colloidal particles. 6.Method according to claim 5 wherein said oxidizing agent is selectedfrom oxygen and a compound of a second metal which becomes incorporatedin the colloidal particles.
 7. Method according to claim 1 wherein step(i) is performed by forming an emulsion having dispersed phase dropletsstabilized by said surfactant and containing a dissolved compound ofsaid first metal and oxidizing said compound of said first metal toproduce said metal-oxo species and form said colloidal particles. 8.Method according to claim 7 wherein said dispersed phase dropletsinclude a dissolved oxidizing agent which is a compound of a secondmetal which becomes incorporated in said colloidal particles.
 9. Methodaccording to claim 1 wherein said hetero atoms are selected from N, B,P, S and O.
 10. Method according to claim 9 wherein said hetero atomsare N.
 11. Method according to claim 1 wherein said metal-oxo species isa metal complex including ligands selected from cyanide, isocyanide,cyanate and isocyanate, thereby acting as a source of carbon andnitrogen as said hetero atom.
 12. Microparticles having a corecontaining a radionuclide encapsulated by a graphitic shell. 13.Microparticles according to claim 12 wherein said core contains, inaddition to said radionuclide, at least one of Fe, Co, Ni and Pd. 14.Microparticles according to claim 12 or 13 wherein said graphitic shellcontains chemically bound hetero atoms such that graphitic layers of theshell are curved.
 15. Microparticles according to claim 14 wherein thehetero atoms are N.
 16. Method of making solid microparticles having ametal-containing core surrounded by a silica coating, including thesteps of (i) forming, in a liquid medium, colloidal particles containinga metal-containing species and colloidally stabilized by a surfactant,and (ii) forming a silica coating around said colloidal particles byhydrolyzing a silicon compound in the region of the interface betweenthe colloidal particle and the liquid medium.
 17. Method according toclaim 16 wherein the colloidal particles contain a plurality of saidmetal-containing species of different metals.
 18. Method according toclaim 16 or 17 wherein the or each said metal-containing species isselected from metal, alloy, metal oxide, metal hydroxide and metalcarbide.
 19. Method according to claim 16 wherein said metal-containingspecies is ferromagnetic and/or contains a radionuclide.
 20. Methodaccording to claim 16 wherein in step (i) said colloidal particles aremade by forming an emulsion having dispersed phase droplets stabilizedby said surfactant containing a dissolved compound of the metal andcausing said metal-containing species to precipitate thereby formingsaid colloidal particles.
 21. Method according to claim 20 wherein theprecipitation of the metal-containing species is caused by addition ofalkali.
 22. Method according to claim 16 wherein the silicon compoundwhich is hydrolyzed is an alkoxy silane compound.
 23. Microparticleseach having a core comprising at least one metal-containing specieswhich is ferromagnetic and/or contains a radionuclide and a coating ofsilica gel encapsulating the core.
 24. Microparticles according to claim23 wherein said metal-containing species is selected from metal, alloy,metal oxide, metal hydroxide and metal carbide.
 25. Microparticlesaccording to claim 23 or 24 wherein said silica gel has at its surfacefunctional groups for the attachment of other species. 26.Microparticles according to claim 23 wherein said silica gel is porous.27. Microparticles according to claim 26 wherein said cores have anaverage diameter in the range 1 to 100 nm, preferably 1 to 50 nm. 28.Microparticles according to claim 27 wherein said coatings have anaverage thickness in the range 1 to 50 nm, preferably 2 to 10 nm.